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I think you will need a crystal ball for this - the weather in September would be very hard to predict.

We had 5 days in Biarritz earlier this month (July) and travelled around the Pyrenees and down into Spain - we experienced every type of weather known to man during those 5 days, hot sun, torrential rain, very strong winds, drizzle ... you will need to wear layers and take an umbrella.

Having said that, it is a lovely area and Biarritz itself a great place to stay.